Here's the thing though. I say this with all the love in my heart, Rhaast isn't that interesting. He's a Fantastic character with great personality, but overall, the dude was just some jerkbag conquering stuff and enslaving people. People are mixing up a fun character, for an interesting character, cause Rhaast is equally, if not possibly greater, a one-note character like Kayn.
Why did Valmar and Kai get their own lore? Because uh, they're still a major factor for the story against the darkin Varus, trying to wrest control from him. They're the focal point around the character, these two lovers forged into one. Their love is the entire reason they currently stand against Varus. If anything, until Riot extremely slightly fleshed out Varus by giving him old Varus lore before he was a Darkin, Varus was the one with no lore and background. Why?
Because the Darkin aren't the main character of these stories. They're antagonists, and while they do need to be fleshed out a little, they are not who the story revolves around. Kayn is the main character of his story, Rhaast is the obstacle between him and his goal. If Rhaast was a deeper character, maybe more lore would be needed, but as he is now, Rhaast is an extremely one-note character. His motivations: To be free and go killing stuff. His past: He was free and killing stuff. His ideal future: To be free and killing stuff, and then maybe getting depressed like Aatrox.
Aatrox has lore, because he doesn't have any host to care about, and hosts become a use and discard aspect of his life. Aatrox is the actual character here, not the conflict between Darkin and Host.